1. Field of the Invention
The present invention relates to a new glaucoma surgical instrument and method, and, in particular, removal of the trabecular meshwork by mechanical cautery, vaporization or other tissue destruction means optionally coupled to an instrument with infusion, aspiration, and a footplate.
2. Description of the Related Art
Aqueous is a clear, colorless fluid that fills the anterior and posterior chambers of the eye. The aqueous is formed by the ciliary body in the eye and supplies nutrients to the lens and cornea. In addition, the aqueous provides a continuous stream into which surrounding tissues can discharge the waste products of metabolism.
The aqueous produced in the ciliary process circulates from the posterior chamber to the anterior chamber of the eye through the pupil and is absorbed through the trabecular meshwork, a plurality of crisscrossing collagen cords covered by endothelium. Once through the trabecular meshwork, the aqueous passes through Schlemm's canal into collector channels that pass through the scleral and empty into the episcleral venous circulation. The rate of production in a normal eye is typically 2.1 μL/min. Intraocular pressure in the eye is maintained by the formation and drainage of the aqueous. All the tissues within the corneoscleral coat covering the eyeball are subject to this pressure, which is higher than pressure exerted on tissues at other locations in the body.
Glaucoma is a group of diseases characterized by progressive atrophy of the optic nerve head leading to visual field loss, and ultimately, blindness. Glaucoma is generally associated with elevated intraocular pressure, which is an important risk factor for visual field loss because it causes further damage to optic nerve fibers. Other causes of glaucoma may be that the nerve is particularly vulnerable to the pressure due to poor local circulation, tissue weakness or abnormality of structure. In a “normal” eye, intraocular pressure ranges from 10 to 21 mm mercury. In an eye with glaucoma, this pressure can rise to as much as 75 mm mercury.
There are several types of glaucoma, including open and closed angle glaucoma, which involve the abnormal increase in intraocular pressure, primarily by obstruction of the outflow of aqueous humor from the eye, or, less frequently, by over production of aqueous humor within the eye. The most prevalent type is primary open angle glaucoma in which the aqueous humor has free access to the irridocorneal angle, but aqueous humor drainage is impaired through obstruction of the trabecular meshwork. In contrast, in closed angle glaucoma, the irridocorneal angle is closed by the peripheral iris. The angle block can usually be corrected by surgery. Less prevalent types of glaucoma include secondary glaucomas related to inflammation, trauma, and hemorrhage.
Aqueous humor is similar in electrolyte composition to plasma, but has a lower protein content. The aqueous humor keeps the eyeball inflated, supplies the nutritional needs of the vascular lens and cornea and washes away metabolites and toxic substances within the eye. The bulk of aqueous humor formation is the product of active cellular secretion by nonpigmented epithelial cells of the ciliary process from the active transport of solute, probably sodium, followed by the osmotic flow of water from the plasma. The nonpigmented epithelial cells of the ciliary process are connected at their apical cell membranes by tight junctions. These cells participate in forming the blood/aqueous barrier through which blood-borne large molecules, including proteins, do not pass.
Intraocular pressure (IOP) is a function of the difference between the rate at which aqueous humor enters and leaves the eye. Aqueous humor enters the posterior chamber by three means: 1) active secretion by nonpigmented epithelial cells of the ciliary process; 2) ultrafiltration of blood plasma; and 3) diffusion. Newly formed aqueous humor flows from the posterior chamber around the lens and through the pupil into the anterior chamber; aqueous humor leaves the eye by 1) passive bulk flow at the irridocorneal angle by means of the uveloscleral outflow, or by 2) active transportation through the trabecular meshwork, specifically the juxta canalicar portion. Any change in 1), 2), or 3) will disturb aqueous humor dynamics and likely alter intraocular pressure.
Primary open angle glaucoma is caused by a blockage in the trabecular meshwork. This leads to an increase in intraocular pressure. The major obstruction is at the juxta-canalicular portion which is situated adjacent to Schlemm's canal. In infants a goniotomy or a trabeculotomy can be performed. In goniotomy or trabeculotomy a small needle or probe is introduced into Schlemm's canal and the trabecular meshwork is mechanically disrupted into the anterior chamber. Approximately 90°-120° of trabecular meshwork can be disrupted. The anatomical difference between congenital glaucoma and adult glaucoma is that in congenital glaucoma the ciliary body muscle fibers insert into the trabecular meshwork and once disrupted the trabecular meshwork is pulled posteriorly allowing fluid to enter Schlemm's canal and to be removed through the normal collector channels that are present in the wall of Schlemm's canal. In adults the trabecular meshwork tears but remains intact and reattaches to the posterior scleral wall of Schlemm's canal blocking the collector channels.
Most treatments for glaucoma focus on reducing intraocular pressure. Treatment has involved administration of beta-blockers such as timolol to decrease aqueous humor production, adranergic agonists to lower intraocular pressure or diuretics such as acetazolamide to reduce aqueous production, administration of miotic eyedrops such as pilocarpine to facilitate the outflow of aqueous humor, or prostaglandin analogs to increase uveoscleral outflow. Acute forms of glaucoma may require peripheral iridectomy surgery to relieve pressure where drug therapy is ineffective and the patient's vision is at immediate risk. Other forms of treatment have included physical or thermal destruction (“cyclodestruction”) of the ciliary body of the eye, commonly by surgery or application of a laser beam, cryogenic fluid or high frequency ultrasound.
In guarded filtration surgery (trabeculectomy), a fistula created through the limbal sclera is protected by an overlying partial thickness sutured scleral flap. The scleral flap provides additional resistance to excessive loss of aqueous humor from the eyeball, thereby reducing the risk of early postoperative hypotony.
In accordance with one recently introduced procedure, a full thickness filtering fistula may be created by a holmium laser probe, with minimal surgically induced trauma. After retrobulbar anesthesia, a conjunctival incision (approximately 1 mm) is made about 12-15 mm posterior to the intended sclerostomy site, and a laser probe is advanced through the sub-conjunctival space to the limbus. Then, multiple laser pulses are applied until a full thickness fistula is created. This technique has sometimes resulted in early hypotony on account of a difficulty in controlling the sclerostomy size. In addition, early and late iris prolapse into the sclerostomy has resulted in abrupt closure of the fistula and eventual surgical failure. Further, despite its relative simplicity, the disadvantage of this procedure, as well as other types of glaucoma filtration surgery, is the propensity of the fistula to be sealed by scarring.
Various attempts have been made to overcome the problems of filtration surgery, for example, by using ophthalmic implant instruments such as the Baerveldt Glaucoma Implant. Typical ophthalmic implants utilize drainage tubes so as to maintain the integrity of the openings formed in the eyeball for the relief of the IOP.
Typical ophthalmic implants suffer from several disadvantages. For example, the implants may utilize a valve mechanism for regulating the flow of aqueous humor from the eyeball; defects in and/or failure of such valve mechanisms could lead to excessive loss of aqueous humor from the eyeball and possible hypotony. The implants also tend to clog over time, either from the inside by tissue, such as the iris, being sucked into the inlet, or from the outside by the proliferation of cells, for example by scarring. Additionally, the typical implant insertion operation is complicated, costly and takes a long time and is reserved for complicated glaucoma problems.
There are many problems, however, in effectively treating glaucoma with long term medicinal or surgical therapies. One problem is the difficulty in devising means to generate pharmacologically effective intraocular concentrations and to prevent extraocular side effects elicited by a systemic administration. Many drugs are administered topically or locally. The amount of a drug that gets into the eye is, however, only a small percentage of the topically applied dose because the tissues of the eye are protected from such substances by numerous mechanisms, including tear turnover, blinking, conjunctival absorption into systemic circulation, and a highly selective corneal barrier.
Pharmacological treatment is prohibitively expensive to a large majority of glaucoma patients. In addition, many people afflicted with the disease live in remote or undeveloped areas where the drugs are not readily accessible. The drugs used in the treatment often have undesirable side effects and many of the long-term effects resulting from prolonged use are not yet known. Twenty-five percent of patients do not use their medications correctly.
Glaucoma is a progressively worsening disease, so that a filtration operation for control of intraocular pressure may become necessary. Present surgical techniques to lower intraocular pressure, when medication fails to decrease fluid flow into the eye or to increase fluid outflow, include procedures that permit fluid to drain from within the eye to extraocular sites by creating a fluid passageway between the anterior chamber of the eye and the potential supra-scleral/sub-Tenon's space, or, alternatively, into or through the Canal of Schlemm (see, e.g., U.S. Pat. No. 4,846,172). The most common operations for glaucoma are glaucoma filtering operations, particularly trabeculectomy. These operations involve creation of a fistula between the subconjunctival space and the anterior chamber. This fistula can be made by creating a hole at the limbus by either cutting out a portion of the limbal tissues with either a scalpel blade or by burning with a cautery through the subconjunctival space into the anterior chamber. Fluid then filters through the fistula and is absorbed by episcleral and conjunctival. In order for the surgery to be effective, the fistula must remain substantially unobstructed. These drainage or filtering procedures, however, often fail by virtue of closure of the passageway resulting from the healing of the very wound created for gaining access to the surgical site. Failures most frequently result from scarring at the site of the incisions in the conjunctiva and the Tenon's capsule. The surgery fails immediately in at least 15% of patients, and long term in a much higher percentage. Presently, this consequence of trabeculectomy, closure of the passageway, is treated with 5-fluorouracil and Mitomycin_C, which apparently prevent closure by inhibiting cellular proliferation. These drugs, however, are highly toxic and have undesirable side effects, including scleral melting, hypotony, leaks, and late infections.
Other surgical procedures have been developed in an effort to treat victims of glaucoma. An iridectomy, removal of a portion of the iris, is often used in angle-closure glaucoma wherein there is an occlusion of the trabecular meshwork by iris contact. Removal of a piece of the iris then gives the aqueous free passage from the posterior to the anterior chambers in the eye. The tissue of the eye can grow back to the pre-operative condition, thereby necessitating the need for further treatment.
In view of the limited effectiveness of treatment options, there is, therefore, a need to develop more effective treatments for glaucoma.